Specifications for conventional underground storage tanks, including those incorporating secondary containment, are identified in the Flammable and Combustible Liquids Code published by the National Fire Protection Association and referred to as ANSI/NFPA 30, an American National Standard. The principal authority for establishing and publishing these tank specifications is Underwriters Laboratories Inc. Until 1964 nearly all underground storage tanks were made of steel and Underwriters Laboratories Inc. originally published only one specification for underground storage tanks: "Standard for Steel Underground Tanks for Flammable and Combustible Liquids, UL 58". On Feb. 2, 1966 a revision of Subject 58 was prepared by Underwriters Laboratories, Inc. to establish performance standards for "nonmetallic" glass-reinforced plastic underground storage tanks. A single wall underground tank meeting those standards, "Nonmetallic Underground Tank for Petroleum Products Only," was identified by Underwriters Laboratories, Inc. on Jul. 7, 1973 under UL File MH 8781. Specifications for making this single wall underground tank are described in Example III of U.S. Pat. No. 3,851,786, issued Dec. 3, 1974.
The 1966 Subject 58 has undergone numerous revisions. In 1977, "Subject 1316" entitled "Standard for Glass-Fiber Reinforced Plastic Underground Storage Tanks for Petroleum Products, UL 1316" was introduced, followed most recently with a revision in 1991 that included the chemical resistance and physical strength performance requirements of a double-wall non-metallic underground storage tank. That tank provides an outer secondary containment capability that prevents a release of the tank contents in the event the inner primary container develops a leak.
When it was recognized that destruction of fresh water supplies and serious damage to the environment resulted from the corrosion of steel underground storage tanks, the U.S. Environmental Protection Agency established corrosion resistance criteria for those tanks. To meet the EPA criteria the NFPA 30 code was modified to include a "Provision for Internal Corrosion," followed by an Underwriters Laboratories Inc. publication dated Nov. 22, 1989 citing another Standard for Safety titled "External Corrosion Protection Systems for Steel Underground Storage Tanks, UL 1746".This standard was revised on Jul. 27, 1993.
Conventional double wall underground storage tanks approved for use in the United States comprise secondary containment in compliance with Underwriters Laboratories, Inc. standards. Steel tanks and nonmetallic tanks having a secondary containment belong to the UL1746 and 1316 categories, respectively.
UL 1746 type tanks having secondary containment usually consist of a plain steel "Subject 58" tank enclosed by a separate fiberglass shell made from a mixture of chopped-strand fiberglass and polyester resin. The UL 1746 tanks generally are not required to meet the same strength or chemical resistance standards as the relatively new UL 1316 type tanks that have a secondary containment capability. Since the inner and outer containers of a double wall UL 1746 tank do not need to resist the same internal test pressure as that required by UL 1316 tanks, they are generally constructed with flat ends rather than domed ends.
Underwriters Laboratories, Inc. has designated six classes of double wall "Subject 1316" type tanks having secondary containment. Three of the classes belong to the designation category referred to as "Type I" secondary containment tanks. Those tanks have an outer shell or cover that does not completely enclose the primary container. The other three classes belong to a second designation category referred to as "Type II" secondary containment tanks. The "Type II" UL 1316 tanks have an outer secondary container that completely encloses the primary container. UL designates the fuels that may be stored in either a Type I or a Type II UL 1316 tank having secondary containment dependent upon the chemical resistance of the tank's primary container. UL 1316 double wall tanks having the least chemical resistance belong to either Class 12 (Type I) or Class 15 (Type II) and are approved for storage of petroleum products only. UL 1316 double wall tanks having the most chemical resistance belong to either Class 14 (Type I) or Class 16 (Type II) and are tested and approved for storage of all petroleum products, as well as all alcohols and alcohol-gasoline mixtures.
The underground storage tanks that comply with Subject 1316 Class 16 (Type II) meet the highest strength and corrosion resistance performance standard established by Underwriters Laboratories, Inc. for the underground storage of flammable and combustible liquids. The primary container (inner wall tank), complying with Subject UL 1316 Class 16 Type II under-ground tank requirements, must be able to resist 25 psi pressure while the outer secondary tank is pressurized to at least 15 psi. The tank must be able to withstand a compression load produced by 11.75 in. Hg vacuum.
The conventional composite storage tanks of the prior art do not meet the 1993 standards of UL 1316 Class 16 (Type II) tanks. For example, the tank described in U.S. Pat. Nos. 3,677,432, and 3,851,786 does not disclose a double wall underground tank composition nor a method of making a composite double wall underground tank that will comply with the new 1993 standards. The double wall structure shown in FIG. 20 of U.S. Pat. No. 3,851,786 is intended to increase the overall section modulus and beam strength of the formed composite structure, rather than provide a secondary container as a back up in the event the inner primary tank leaks. That construction does not illustrate how such a composite structure can be adapted to provide underground tanks having secondary containers with provisions for annulus access of leak detection sensors and pressure-resistant tank outlets. Example III of U.S. Pat. No. 3,851,786 details the construction of a single wall underground tank that complied with 1973 UL test requirements established for nonmetallic underground tanks used only for the storage of petroleum products. The conventional laminate construction used to fabricate the single wall underground tank described in Example III of U.S. Pat. No. 3,851,786 does not meet the chemical resistance requirements outlined in the revised (1987) UL Subject 1316 for nonmetallic underground tanks used to store alcohol and petroleum products.
The prior art does not disclose a method for making a double-wall composite tank laminate structure having a wall thickness of only 0.12 inches (3 mm), that is able to pass the extensive series of current UL 1316, Class 16, Type II physical and chemical resistance tests. As is well known, the laminate thickness is a principal factor in determining the double-wall tank manufacturing cost and thus the ability to reduce thickness and yet maintain chemical and physical resistance is desirable.
All other conventional double-wall underground tanks currently listed under UL 1316 for storage of alcohol, gasohol and petroleum products are dome-ended cylinders made from a mixture of chopped strand fiber-glass and a thermosetting polyester resin. In order to comply with NFPA 30, the Flammable and Combustible Liquids Code of the National Fire Protection Association, those prior art all-fiberglass underground tanks must meet the structural and corrosion resistant requirements outlined in UL 1316 and are tested to demonstrate an ability to resist an internal pressure of 25 psi (172 Pa) and a compression load equal to that produced by a negative pressure (vacuum) of -6 psi (-41 Pa). Unlike the flat-ended UL 58 steel underground storage tanks that can not safely resist a test pressure exceeding 5 psi, all approved non-metallic underground tanks must meet the pressure strength requirement of 25 psi with a factor of safety of 5. For that reason, all large diameter UL 1316 underground tanks must be fabricated as pressure vessels having hemispherical tank ends.
Prior art UL 1316 type double-wall all-fiberglass underground tanks that for the past 30 years have been adopted as an industry standard are still made from two chopped-strand fiberglass tank half-shells that are joined at the tank mid-section with resin-impregnated fiberglass cloth that overlaps the abutting edges of each tank half-shell. Each of those half-shells are made on a two-piece collapsible or removable steel mandrel upon which a mixture of chopped fiberglass and polyester resin is applied. The removable mandrel upon which each tank half-shell is made is shaped to form the domed end as well as half of the tank's cylinder. In some cases, the tank half-shell mandrel is supported at one end by a powered axle that acts as a rotating cantilever beam.
A conventional method for making a double-wall fiberglass tank half-shell involves the steps of placing a resin-release agent upon a half-shell mandrel surface, applying a mixture of polyester resin and chopped strand fiberglass upon the tank half-shell mandrel to make a tank inner wall structure, placing fiberglass rib formers on the half-shell inner wall, spraying a thin coat of resin-wet chopped strand fiberglass upon the rib formers, curing the half-shell inner wall material, perforating the sides of each fiberglass rib at several locations, placing a resin-release annulus-forming film on the inner wall tank head and a cylindrical portion of the tank inner wall between (but not on) each of the fiberglass ribs, and spraying a mixture of polyester resin and chopped strand fiberglass on the inner wall tank heads and the ribbed inner wall cylindrical portion to provide the double-wall tank half-shell with a secondary containment capability. The tank half-shell is then removed from the mandrel, placed on a cart and moved to a cut-off saw that precisely trims the shell so its edges can be matched with those of a second tank half-shell to which it is permanently bonded by an overlapping strip of resin-wet fiberglass cloth.
Conventional UL 1316 double-wall nonmetallic underground tank structures made from chopped strand fiberglass and a thermosetting resin possess a low tensile modulus and consequently are inherently flexible structures that will ovalize, change shape and possibly fracture unless they are carefully installed in and surrounded by pea gravel, crushed rock or other highly compacted soil. It is known in the art that each chopped strand of fiberglass material contains hundreds of short dry glass filaments that are tightly glued together by a starch binder to enable the strand of continuous glass filaments to be cut by the rotating razor blades of a strand-dispensing chopper gun. It is also well known in the art that the polyester resin mixed with the chopped strands of fiberglass does not completely dissolve the starch binder. For this reason the chopped strand fiberglass material used to make prior art underground tank structures contains millions of tiny dry-filament bundles surrounded by polyester resin. These dry filament bundles behave as micro-fractures in the resin matrix that reduce the tensile modulus of the fiberglass tank material. The use of dry sand in the construction of conventional chopped-strand fiberglass tanks provides another source of micro fractures and structural strength uncertainty. For this reason the resin-coated chopped strand fiberglass material comprising prior art double-wall nonmetallic underground storage tanks fails to provide the long term reliable leak-proof corrosion-resistant structural material desired by users of underground fuel storage tanks.
Conventional procedures used to make double-wall fiberglass underground tanks employ expensive and troublesome removable mandrels that require special care in their use and storage, as well as frequent maintenance and repair. The rate of tank production depends upon the availability of the removable tank mandrels. For this reason conventional fiberglass tank half-shells must be removed from the tank mandrel as quickly as possible. The tank half-shell removal time, however, is a function of the shell material cure time. Unfortunately, due to the presence of a wide variety of production variables, the material cure time of prior art fiberglass tank half-shells becomes extremely difficult to accurately predict or control. For example, the fabrication of conventional fiberglass tank half-shells greatly depends upon the skill, temperament and fatigue of the person responsible for controlling the quantity, ratios and placement of the chopped strand fiberglass and resin materials. Furthermore, the complexity of computer-controlled mandrel and carriage equipment used to make conventional fiberglass tank half shells is a cause of frequent production interruptions. The daily changes in ambient temperature and humidity require concomitant changes in the proportions of promoter and catalyst added to the polyester resin matrix used to make conventional fiberglass tank half-shells. The use of electrical heaters to accelerate the cure and hardening of the polyester resin used to make prior art fiberglass tank half-shells also requires special care to prevent the resin matrix from becoming too hot or igniting and burning. The manufacture of conventional fiberglass tank half-shells requires that the weight consumption of each of the materials as well as the thickness of the tank half-shell head, dome and ribs be continually measured and recorded to provide the necessary quality control. Mandrels used to make conventional fiberglass tank half-shells must be continually rotated until the chopped strand fiber-glass material cures thereby preventing the wet tank half-shell material from sliding off the mandrel onto the floor. If, due to the pressure of time and production goals, a conventional fiberglass tank half-shell is removed from the mandrel too soon, it will ovalize and become out of round, making it difficult to trim and match with another fiberglass tank half-shell. The polyester resins used to manufacture most conventional fiberglass underground tanks are isophthalic polyester resins that do not contain a styrene suppressant additive. Since these polyester resins usually contain a weight percent of 40 to 50% of styrene monomer the manufacture of prior art all-fiberglass tank requires the use of expensive equipment to control the air pollution that results from the requisite spraying operations. The safe disposal and handling of the substantial quantity of flammable scrap materials resulting from fiberglass overspray and such operations as sawing, trimming, and flushing resin transfer lines, are additional concerns associated with the conventional production methods and apparatus used to make the conventional double-wall nonmetallic underground storage tanks in compliance with UL 1316 standards.